It is well-known to secure a motor vehicle against operation by a person who has consumed alcohol by installing in said vehicle a breath alcohol tester. A typical arrangement, known as an ignition interlock device (IID), involves a relay between the breath tester and the starter motor, pursuant to which the starter motor cannot be engaged until a satisfactory breath sample has been given. In this regard, a ‘satisfactory’ gas sample is conventionally understood to be one that (I) is of sufficient volume and pressure to permit alcohol analysis; (II) has alcohol concentration below a predetermined limit; and (III) appears to have originated from the exhaled breath of a human being. Criterion (III) is often assessed through measurements of pressure, humidity and temperature, but various other techniques such as hum recognition sensors are occasionally used. In some jurisdictions, a person convicted of driving under the influence of alcohol may be required by law to have a device of this type installed as a condition associated with the extension of driving privileges.